Bone endures large stresses from habitual and sporadic loading. Under normal circumstances, bone will withstand these stresses without damage. However, if the bone is weak (e.g. from osteoporosis) or damaged (e.g. from tumors invading the bone wall or preexisting fractures), or if a portion of the bone is to be replaced by an implant, the bone or remaining bone may be too weak to withstand these stresses. Therefore, a biomechanical analysis of the bone (or bone-implant system) can be a valuable aid in deciding the best future course of treatment for the patient. For example, if the bone is likely to fracture under the stresses of normal living, more aggressive treatment of osteoporosis would be indicated. For another example, analysis could indicate whether an implant or pinning is required after removal of a tumor invading the bone wall, or whether bone cement would be sufficient, or, possibly, that no structural infill is needed to retain the structural integrity of the bone.
In the case of replacing a portion of the bone by an implant, implants sometimes fail or loosen and sometimes this leads to fracture of the bone to which they are attached. Biomechanical analysis of a bone-implant system could be used clinically to improve surgical planning. The information derived from such analyses could aid the surgeon in choosing an optimal size and position of a given implant design for a particular patient, or even choose among different implant designs.
Biomechanical analysis could also be used during the design of implants to improve the design, by providing in-computer analysis of the response of both the bone and the implant in bone-implant systems. At the design stage, both patient-specific and generic bones can be used for the in-computer analysis whereas, at the clinical stage, patient-specific bones are preferable.
However, a system such as described above requires software capable of automatically transforming an image of the bone, such as a CT scan, into a finite element (FE) mesh, and of automatically analyzing the stresses, strains and displacements which are the result of a finite element analysis (FEA) in order to provide the clinician with the information he needs—the likelihood of success or failure of a specific intervention.
U.S. Pat. No. 8,126,234 to Edwards discloses to a method and system for orthopaedic surgical planning and more specifically to surgical planning based on an automated FEA of a bone-implant system using a 3D medical image of a patient. However, U.S. Pat. No. 8,126,234 requires the use of a mask for generating the finite element mesh, whereas no mask is needed in the method of the present invention. Furthermore, the FE mesh of U.S. Pat. No. 8,126,234 is generated by overlaying the mask described hereinabove with a predetermined FE mesh which includes both the implant and the bone, whereas, in the present method, the bone mesh is determined ab initio for each bone analyzed.
Furthermore, U.S. Pat. No. 8,126,234 is based on pre-meshed non-patient-specific models that are made patient specific by adding layers of elements where they are needed. It is claimed in U.S. Pat. No. 8,126,234 that to mesh ab initio for each individual patient from captured geometry requires too much FEA expertise and is too time consuming. Because of the high degree of variability of bone geometry in the population, for ab initio meshing, human intervention would be required to adjust the mesh to avoid badly distorted elements. Therefore, these problems make it infeasible to use the method of U.S. Pat. No. 8,126,234 in a clinical setting.
According to U.S. Pat. No. 8,126,234, fully automatic meshing ab initio results in meshed models containing a large enough number of low-order finite elements that solving such meshes becomes infeasible on the timescales required in a clinical context.
It should be noted that although in U.S. Pat. No. 8,126,234 the material properties are estimated from the grey scale of the scan, no details are given of the process by which they are determined. Care must be taken in converting grey scale data to material properties, as inaccurate material properties can invalidate the results.
Furthermore, U.S. Pat. No. 8,126,234 does not address the numerical accuracy of the results, nor the validation of these by past experiments, or describe any means by which errors can be assessed or controlled.
In addition, I/O for U.S. Pat. No. 812,623 is via a conventional terminal or monitor.
It is therefore a long felt need to provide a fully-automatic system that can accept an image of a bone, extract therefrom the bone's shape in three dimensions and the bone's density as a function of position in three dimensions, create and analyze strains/stresses on the physiologically-loaded bone, with or without implants or other surgical modifications, assuring the accuracy of the results while accepting input and displaying results on a hand-held device.